Abstract: PURPOSE: No systematic study has analyzed the changing patterns of use of postmastectomy radiation therapy (PMRT) during the period of information dissemination regarding the benefit of radiation therapy in this setting. We sought to study the receipt of PMRT in elderly women in this period, using a population-based cohort of women with breast cancer. PATIENTS AND METHODS: Using data from the linked Surveillance, Epidemiology, and End Results-Medicare database, we analyzed the use of radiation therapy between 1991 and 1999 in 19,699 women with stage I to III breast cancer who received mastectomy as definitive surgery. Multivariable logistic regression was used to investigate the association of diagnosis year and the use of PMRT, after controlling for clinical and sociodemographic factors. RESULTS: During the entire period studied, 2,160 (11.0%) patients treated with mastectomy received PMRT. The use of PMRT significantly increased in women diagnosed in 1996 (odds ratio [OR], 1.26; P = .04), 1997 (OR, 1.70; P < .0001), 1998 (OR, 1.57; P < .0001), and 1999 (OR, 1.77; P < .0001), relative to those diagnosed in 1991 after controlling for cancer, and sociodemographic and treatment characteristics. There were significant differences in the temporal trends of radiation use among different regions of the country (P < .0001), and between teaching versus nonteaching institutions (P = .04). CONCLUSION: The use of PMRT in elderly women increased significantly during the period of data dissemination about its efficacy; however, the trends in adoption varied among different practice settings.